Apple’s iBookstore is now integrated into the iTunes Store. You can find its link at the top of the iTunes window between “App Store” and “Podcasts.”
The book categories includes the enhanced books which includes some children’s books with human narration in a read-along mode. The bookstore includes a selection of $2.99-or-less books as well as two free books (listed below).
I bought a copy of Robert Charles Wilson’s SF novel “Spin” for $2.99 to test the purchase process and found it quick and painless. The novel was a fast 1.4MB download. I placed it on my iPhone 4 and my iPad 2. Unlike Amazon’s Kindle (and its apps), over-the-air page syncing was performed immediately without asking for permission. Some people will find this useful and welcomed (I’m in that camp). Others may be taken aback. There does not appear to be an option in settings to turn this off.